An organic EL element is an emissive element using a principle that a fluorescent material emits light by means of recombination energy of holes injected from an anode and electrons injected from a cathode when an electric field is applied thereto. Researches have been conducted with regard to the organic EL element comprised of an organic material after C. W. Tang and S. A. VanSlyke of EASTMAN KODAK, Co. reported a low voltage driving organic EL element using a stacked device in Applied Physics Letters, Vol. 51, pp. 913 (1987).
Tang et al fabricates the organic EL element using tris(8-quinolinol)aluminum for an emitting layer and triphenyldiamine derivative for a hole transporting layer on a glass substrate. Such stacked type structure has advantages in that efficiency of injecting the hole into the emitting layer may be increased, efficiency of creating excitons resulted from recombination by blocking electron injected from the cathode may be increased, and the excitons created in the emitting layer may be trapped. For the structure of the organic EL element as mentioned above, a two layered type consisting of a hole transporting (injecting) layer and a hole transportable emitting layer, or a three layered type consisting of a hole transporting (injecting) layer, a light-emitting layer and an electron transporting (injecting) layer are well known in the art.
Researches for the element structure or a method for forming the same are conducted in order to increase the recombination efficiency of the injected electron and hole in the stacked type element.
However, the organic EL element has a limit for the probability of singlet generation due to dependency of spin statistics when carriers are recombined, which causes to have an upper limit of light emitting probability. The value of such upper limit is expected to be about 25%. In addition, the light of exiting angle larger than critical angle is fully reflected due to the refractive index of the light emitting body in the organic EL element, so that the light may not be extracted outside the substrate. It is expected that only 20% of the total amount of light emitted is available when the refractive index of the light emitting body is 1.6, As such, energy converting efficiency becomes inevitably low, which is limited to be about 5% (0.25×0.2=0.05) in total even when singlet creation probability is added to, which was reported by Tsutsui Tetsuo in ┌phenomena and trend of organic electroluminescent device┘, Monthly Display, Vol. 1, No. 3, pp. 11, September (1995). In the organic EL element that has a light emitting probability highly limited, low light extraction efficiency as mentioned above causes degradation of the energy converting efficiency, which may be severely and adversely affected.
As for the technique of improving the light extraction efficiency, several proposals have been suggested using the light emitting diode having the same structure as that of an inorganic EL element in the prior art. For example, JP patent laid-open publication No. 1988-314795 discloses a method for forming a lens on a substrate with a focusing property to improve the efficiency, which is efficient for a device having a large emitting area, however, it is difficult to form the lens with the focusing property in a device such as a dot matrix display device in which each pixel area is fine. In addition, JP patent laid-open publication No. 1987-172691 discloses a method for forming anti-reflective layer by introducing a planarizing layer that may have a medium refractive index between the glass substrate and the light-emitting layer, which improves the light extraction efficiency for the front direction, however, may not effectively prevent total reflection. Thus, it is not effective for the organic EL element having a relatively low refractive index, but effective for the inorganic EL element having a high refractive index.
In addition, JP patent laid-open publication No. 2000-231985 discloses a method for reducing total reflection at a plane not in contact with the organic EL element of the substrate by using the technique that the plane has a function of diffusing light, however, its effect is not significant because the rate of light totally reflected is high at an interface between the organic EL element and the glass substrate that has been conventionally used.
In addition, when a light emitting diode where a plurality of organic EL elements are arranged is fabricated using the substrate with such light diffusion function, light emitted is from the organic EL element reaches pixel regions adjacent to the element, which causes the light to be leaked and observed even at a non-emitting pixel. JP patent laid-open publication No. 1999-8070 discloses a technique for forming a black mask and a light diffusing layer between the substrate and the organic EL element to solve the light leakage problem.
However, some of the lights are absorbed by the black mask so that the light extraction efficiency is further degraded.
As such, the method for improving the light extraction efficiency and preventing the light leakage of the light emitting diode using the organic EL element is not sufficient, and the above mentioned problem must be inevitably overcome to put the organic EL element to practical use.